This invention relates to display systems and more particularly to restricted area display units such as hand holdable data terminals.
In a known terminal, having a display area capable of accepting a particular maximum number of lines of data, the lines of data to be displayed are placed upon the screen in consecutive fashion until the total line availability of the display screen has been filled. After this condition has been attained it is necessary to provide some form of control--commonly called rackup for shifting the display with respect to the screen, conventionally in an upward direction, line-by-line so as to make available line space for receiving and displaying further lines of data. It is desirable that the lines of data racked out of display view be stored so as to allow for the return of previously displayed lines to the viewable display area. This requirement becomes more pressing particularly with hand held interactive terminals, according to the degree by which the amount of data in use exceeds the display capacity and is satisfied by providing a screen buffer with a capacity larger than the display screen capacity. Bearing in mind that it is desirable for a hand held interactive terminal having a display of the order of four lines to be able to imitate as far as possible conventionally sized display screens it is important that such a screen buffer is provided which is able to accommodate at least the maximum number of lines likely to be expected by an application of the display. For example, an overall area equivalent to the line possibility of an A4 sheet--that is some sixty four possible data lines. Again, bearing in mind that a hand holdable terminal can often display up to four lines only, it is necessary that the screen buffer should be able to accommodate scroll-back of all lines which have been racked out of sight but which still remain within the above mentioned A4 screen size. Furthermore, since a hand held terminal is frequently used off line in the manner of a note pad, (in that data is entered which it is desired to retain for periods which may overlap, partially or entirely a line activity) it is clearly desirable to be able readily to remove from the screen buffer any trivial data or transient data it is not necessary to retain.